1. Cross Reference to Related Application
This application is one of four related applications, Ser. Nos. 09/566,445 , 09/566,930, 09/568,720 and 09/569,938.
2. Field of the Invention
The present invention relates to a charging controller, and more particularly to a charging controller for charging a secondary battery used in a portable electronic device, for example.
3. Description of the Related Art
Portable electronic devices such as a notebook-sized computer, and so forth contain a secondary battery. As to a charging controller for driving a unit and charging the secondary battery simultaneously, the following techniques have been disclosed.
According to a first technique, the rating of output power from a dc source is designed so as to be not less than the sum of the maximum consumption power of a unit and maximum charging power.
According to a second technique, as described in Japanese Unexamined Patent Publication No. 5-137276, only a unit load current is detected, the difference between the rated current of a dc source and the unit load current is determined, and a charging output is controlled so that the charging current is equal to the difference.
According to a third technique, as described in U.S. Pat. No. 5,723,970, an output current from a dc source is detected, and a charging output is controlled so that the output current from the dc source is prevented from exceeding the rated current.
Generally, a portable electronic device can be driven by application of power which is considerably lower than the maximum consumption power, and the time period while the device is required to be driven by application of the maximum consumption power is very short. FIG. 5 shows the operation conceptual diagram. In FIG. 5, the maximum power required to drive the unit is 30 W, and the maximum charging power is 30 W. Accordingly, the supply capability of the dc source is 60 W. However, since the power required to drive the device is changed with time, a large part of the power supply capability of the dc power is surplus.
For this reason, regarding the above-described first technique of the conventional example, it is needed that the power supply capability of the dc source is designed so as to be surplus to a unit consumption power required under ordinary operation. Thus, there arises the problem that the shape and size and the cost of the dc source are increased.
In the above-described second technique of the conventional example, the charging output is controlled so that the rated current of the dc source is not exceeded. Therefore, the maximum supply power of the dc source can be reduced to the maximum consumption power of the unit. However, since the charging current is kept constant, irrespective of the charging voltage, the surplus power of the dc source can not effectively be used.
FIG. 6 illustrates the relation between the charging voltage and the charging current, obtained in the second technique of the conventional example. The rated power of the dc source is 20V/1.5 A (30 w), the unit current draw is 1.2 A, and the charging voltage range is 9.0-13.0V.
FIG. 7 shows an output power characteristic. According to the second technique of the conventional example, the charging current is kept constant, irrespective of the charging voltage as described above. However, the power available for charging (20 W.times.1.5-20 W .times.1.2=6 W) is constant, as shown in FIG. 7. Accordingly, if the charging is carried out in this technique, the power supply capability of the dc source becomes surplus when the charging voltage is low.
Accordingly, in the second technique of the conventional example, the power supply capability of the dc source can not effectively be used. Thus, there arises the problem that the charging time is increased, in spite of the power supply capability of the dc source.
FIG. 8 is a circuit block diagram of a charging controller which employs the above described third technique of the conventional example. In FIG. 8, an output 15 from a dc source 1 is provided to a charging control circuit 2 via a dc source output current detection resistor 5 of a charging circuit 14, and also provided to a DC-DC converter 6 via a rectification element 13. The input 17 of the DC-DC converter 6 is connected to the anode of a secondary battery 4 via a rectification element 12. The output 18 of the DC-DC converter is connected to a unit load 7 of a portable device.
A voltage developed across the dc source output current detection resistor 5 is detected by a dc source output current detection circuit 10, and the detection signal 21 is provided to a control circuit 2 of the charging controller. The output of the charging control circuit 2 is connected to the anode of the secondary battery 4 via a charging current detection resistor 3. A voltage developed across the charging current detection resistor 3 is detected by a charging current detection circuit 8, and the detection signal 19 is provided to the charging control circuit 2. A charging voltage on the anode side of the secondary battery 4 is detected by a charging voltage detection circuit 9, and the detection signal 20 is also provided to the charging control circuit 2.
Next, the operation of the charging circuit 14 shown in FIG. 8 will be described. In the case of charging while the unit stops, a voltage developed across the charging current detection resistor 3 is detected by the charging current detection circuit 8. The charging voltage detection circuit 9 detects a charging voltage on the anode side of the secondary battery 4. The output 19 from the charging current detection circuit 8 and the output 20 from the charging voltage detection circuit 9 are fed back to the charging control circuit 2, whereby constant-voltage, constant-current charge is carried out.
When the unit is under operation, a voltage developed across the dc source output current detection resistor 5 is detected by a dc source output current detection circuit 10, and the detection output 21 is fed back to the charging control circuit 2, whereby the charging output is controlled so that the output current from the dc source 1 is prevented from exceeding a predetermined value.
With the charging controller shown in FIG. 8, the surplus power determined by subtracting a practical unit consumption power from the current supply capability of the dc source 1 can be utilized as charging output power, without any surplus or shortage. Accordingly, the charging time of the secondary battery 4 can be reduced.
However, it is needed that all of the current required for driving the unit and charging is made to flow through the dc source output current detection resistor 5 for detecting the output current from the dc source 1. Therefore, a loss and heat generated in the dc source output current detection resistor 5 are increased, the reliability of the circuit is reduced, and the current detection accuracy deteriorates, caused by effects on the temperature characteristic of the dc source output current detection resistor 5. Moreover, it is needed to take heat dissipation measures such as attachment of a radiation plate, which causes the problem that the shape and size, and the cost of the charging circuit are increased.